A method of treating tumors includes radiation therapy which irradiates an affected part with radiation. In the radiation therapy, it is preferable that a radiation dose (a dose) for a patient be reduced as small as possible and an affected part be efficiently irradiated with radiation. For this reason, stereotactic radiotherapy through which an affected part of a patient can be irradiated with radiation in multiple directions has been used.
As shown in FIGS. 8 and 9, a constitution including an annular support frame 1 which is provided to surround a body axis of a patient, annular rails 2 which are provided on lateral surfaces of the support frame 1, and an annular gantry 4 which is pivotable about a horizontal axis along the rails 2 and has a radiation irradiating unit 3 is disclosed in Patent Literature 1 as an apparatus which performs such stereotactic radiotherapy. In such a radiation therapy apparatus, the gantry 4 is pivoted along the rails 2 so that the patient is irradiated with radiation in multiple directions by turning the radiation irradiating unit 3 installed at the gantry 4 about the body axis of the patient.